As integrated circuit devices shrink, and as circuit speed and performance increase, copper has replaced aluminum as the preferred electrical interconnect material. The use of copper as an interconnect material in silicon integrated circuits has occurred in response to the need for lowered interconnect resistivity, good electromigration resistance, and good deposition characteristics which allow effective filling of vias and contacts.
Copper metallization structures are often formed by a process known as Damascene, wherein an interlevel dielectric layer (ILD) has regions etched therein into which metal lines will be inlaid. A barrier layer is deposited, which serves to prevent diffusion of copper from the metal lines into the dielectric. This barrier layer is generally comprised of Ta or Ta compounds. A copper seed layer is then generally deposited, followed by an electroplated copper layer. The excess copper is then removed by a process known as Chemical Mechanical Polishing (CMP). CMP enhances the removal of surface material over large distances and short distances by simultaneously abrading the surface while a chemical etchant selectively attacks the surface. For this purpose, CMP typically utilizes a polishing slurry containing both an abrasive and a chemically active component.
A problem in standard prior art CMP, particularly during the overpolishing necessary to remove all the metal (or other material being polished) from the surface, is known as recession or dishing, which is illustrated in FIG. 1. Uneven wafer surface 2 has recessed region 4. Deposited layer 6 is polished off of the surface, but in the center 8 of region 4, the surface of the polished deposited layer 6 is at a lower level than at the edge 10 of region 4. The dishing effect is more serious for wide shallow recessed regions than for narrow deep recessed regions.
A prior art method for reducing dishing during CMP overpolish is the fixed-abrasive method, whereby abrasive particles are suspended in a stationary medium on the pad rather than being incorporated into the slurry. The free-floating abrasive particles which are the primary cause for dishing are not present using this method. Fixed-abrasive CMP is described by E. Korczynski in “A Level Future”, Semiconductor Magazine, Vol. 3, No. 7, July 2002.
Fixed-abrasive CNIP has its own associated set of problems. The fixed-abrasive CMP system's polishing pads are generally 5–10 times more costly than standard polishing pads. Additionally, there is a balancing act between throughput and low density of scratches: in order to achieve a high polishing rate, the fixed abrasive particles cause appreciable scratching. Also, there may be some residual dishing due to large abrasive particle size. Finally, the fixed abrasive pad will degrade during use, which leads to variable polish rates and amount of micro-scratches.
A CMP method for polishing metals such as copper that reduces dishing effects and achieves a high polish rate and low density of defects and scratches would be an important improvement in CMP technology.